1. Field of the Invention
The invention is related to electron beam welding and, in particular, to electron beam welding of downhole tools for use in oil well applications.
2. Description of the Prior Art
Downhole oilfield tools often consist of complicated "blocks" that are small in diameter and long in length. These "blocks" have previously been joined by the conventional "pin and box" joint well known to those skilled in the art. This joint is undesired since it adds several inches to the tool length.
Electron Beam Welding processes have been in use for quite some time. See, for example, Benedict, Gary F. "Nontraditional Manufacturing Processes" (1987), Chapters 18 and 19. Electron beam welding is a fusion welding process in which heating results from the impingement of a beam of high velocity electrons on the metal to be welded. Originally developed for obtaining ultra-high-purity welds in reactive and refractory metals, its unique qualities have led to substantial use in numerous operations.
The electron optical system for the process starts with a high voltage current heating a tungsten filament to about 4000.degree. F. causing it to emit high velocity electrons. By means of a control grid, accelerating anode and focusing coils, the electrons are formed into a concentrated beam and focused onto the workpiece in a spot measuring about 1/32 to 1/8 inch in diameter. Since electrons cannot travel well through air, the beam must be generated and focused in a very high vacuum.
In most operations, the workpiece is also enclosed in the high-vacuum chamber and must be positioned and manipulated in this vacuum. In some applications, however, it is possible to have some portion of the workpiece positioned outside the chamber if it can be sealed for vacuum purposes. High-vacuum electron beam welders are defined as those that operate with the workpiece at a pressure ranging from 0.13 to 1.3.times.10.sup.-4 Pa (10.sup.-3 to 10.sup.-6 torr). When welding under these conditions, the vacuum assures degasification and decontamination of the molten weld metal, and very high quality welds are obtained. Materials that are difficult to weld by other processes, such as zirconium, beryllium and tungsten, can be welded successfully by electron beam welding, but the weld configuration should be simple and preferably flat. The high power and heat concentrations can produce fusion zones with depth-to-width ratios of 25:1 with low heat input, low distortion and a very narrow heat-affected zone. Heat-sensitive materials can be welded without damage to the base metal. Additionally, high welding speeds are common, no filler material is required, the process can be performed in all positions and preheating or postheating is generally unnecessary.
The process of electron beam welding through holes was first developed by Applicant for a Slim Repeat Formation Tester project in 1992. The process was utilized to weld a tubular end onto a part which had holes originating at the intersection. (See FIGS. 1A and 1B) These holes were sealed by first installing a plug which was then electron beam welded through. In this process, however, removal of the plugs was not required. A test piece was designed to demonstrate the capabilities of this procedure and is described below with reference to FIG. 1.